SUMMARY: 1) Spontaneous and carcinogen induced cancers in mice and rats were prevented by a variety of viral and tumor cell vaccines featuring species specific oncogene (sarc and leuk) transforming proteins; in both species tumor antigens were held in common with antigens expressed by src and leuk oncogenes; 2) passive immunity to mouse leukemia and sarcoma viruses provided solid protection against spontaneus leukemias and cancers produced by chemical carcinogens; 3) protective rat tumor cell vaccines specifying src or leuk sequences were isolated from chemically induced cancers; 4) innovative tumor cell vaccines involving histoincompatible mink cells bearing mouse or rat sarc transforming proteins gave profound protection against chemical carcinogens; 5) chimpanzee and human tumor cell vaccines are being developed for future efforts to prevent cancer.